With PostGIS, I need to perform the union of two polygon layers that have intersection areas and distinct areas while preserving all limits. I need to extract separated polygons for each intersected and distinct areas from both layers.

Using QGIS, the tool is called "Vector > Geoprocessing > Union"

here is a visual of the problem:

Two Input polygon layers (green vs red dashes) = Two Input polygon layers

Output polygon layer (one distinct color per polygon)= Output polygon layer

I need to do it with PostGIS. st_union seems to do a different job. Any idea?

  • 1
    You need to split or to join polygons (dissolve boundaries) with intersections? Seems to be splited: we can first to isolate parts with ST_Intersection() and ST_Difference(), them scan all with usual SQL UNION, them merge all in a layer by ST_Collect() (faster tham ST_Union). Commented Nov 9, 2018 at 22:30

1 Answer 1


You can try something like this.

select distinct a.geom1 from
(select distinct(st_dump(st_collect(t1.geom,t2.geom))).geom as geom1
 from t1 inner join t2 on not st_intersects(t1.geom,t2.geom)) a inner join 
(select distinct (st_dump(st_collect(st_symdifference(t1.geom,t2.geom),st_intersection(t1.geom,t2.geom)))).geom
from t1 ,t2 where st_intersects(t1.geom,t2.geom)) b
on not st_intersects(a.geom1,b.geom)
select (st_dump(st_collect(st_symdifference(t1.geom,t2.geom),st_intersection(t1.geom,t2.geom)))).geom
from t1 inner join t2 on st_intersects(t1.geom,t2.geom)

Note: t1 and t2 are names of tables containing polygon geometry and name of geometry column in my case is geom in both tables, it may be different in your case.

  • YES it works. st_symdifference is however pretty slow. Thanks
    – Martin C
    Commented Nov 10, 2018 at 9:21
  • 1
    Then please accept this answer
    – Asad Abbas
    Commented Nov 10, 2018 at 9:31

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